Carbon nanotubes have a reputation for being strongest possible fibers, but polyyne chains are stronger, as measured by the critical strength/density ratio: Polyyne carbon-carbon bonds are stronger than the bonds in graphene and nanotubes, and the bonds are all are aligned with the axis of the fiber, the optimal geometry for carrying tensile stress. A paper in Nature Chemistry reports the longest polyyne chain synthesized to date, a chain of 44 carbon atoms.
Polyyne structures look like the middle section of the diagram to the right. From a tensile-strength point of view, this standard chemical diagram (which shows alternating single and triple bonds) is somewhat deceptive: What are formally “single bonds” (and therefore might be judged weak) are uncommonly short and strong: < 0.139 nm vs. > 0.142 nm for the bonds in graphene and carbon nanotubes (which have a formal bond order of 1.33).
I’m tempted to cite ~ 0.131 nm for the length of the polyyne bond, but as noted in a recent paper, conventional density-functional calculations are poor at reproducing geometries affected by the Peierls instability, and bond-length alternation in polyynes is a textbook example of it. (Fortunately, this also dampens my urge to run a decent DFT calculation on a polycumulene-like structure that would maintain strong bonding through the interface to an end-group, a cyclopropane. A quick look was interesting, but the interesting parts are linked to the Peierls instability.)
There are reports of a carbon allotrope, “carbyne”, that is (or would?) consist entirely of polyyne chains, but last time I checked (as I do every few years) these reports haven’t described reproducible, well-characterized materials. Carbyne allotropes of carbon are interesting from an advanced-technology perspective, since materials with extreme properties are often useful in engineering, but for now, the instability of polyynes in the laboratory places such stuff outside the scope of conservative exploratory engineering methodologies.
Note the extraordinarily bulky groups at the ends of the newly reported polyyne molecule. They are there for a reason, and the reason is stability.